Acrylic rubber composition, crosslinkable acrylic rubber composition, and crosslinked object obtained therefrom

ABSTRACT

An acrylic rubber composition comprising 100 parts by weight of (a) an acrylic rubber having carboxyl groups, 0.3 to 8 parts by weight of (b) an aromatic secondary amine compound, and 0.3 to 8 parts by weight of (c) nickel dialkyldithiocarbamate. A crosslinkable acrylic rubber composition further comprising 0.1 to 10 parts by weight of a crosslinking agent in addition the ingredients (a), (b) and (c). By crosslinking the crosslinkable acrylic rubber composition, a crosslinked article having high heat resistance and exhibiting especially a reduced variation in elongation by heat load can be obtained.

TECHNICAL FIELD

This invention relates to an acrylic rubber composition that is amaterial for giving a crosslinked article having excellent heatresistance; a crosslinkable acrylic rubber composition; and acrosslinked article made by crosslinking the crosslinkable acrylicrubber composition. More specifically, it relates to in acrylic rubbercomposition and a crosslinkable acrylic rubber composition that is amaterial for giving a crosslinked article exhibiting a reduced variationin elongation upon the imposition of heat load; and a crosslinkedarticle made by crosslinking these rubber compositions.

BACKGROUND ART

Acrylic rubbers have good heat resistance, and therefore, are widelyused in an automobile field and related fields.

However, there has been an increasing demand for producing an acrylicrubber having more enhanced heat resistance. To meet this demand, anattempt of increasing the amount of an antioxidant incorporated in therubber was proposed in the early stage. This proposal invited a problemsuch that, when a large amount of an antioxidant was incorporated, bloomoccurred on crosslinked articles. Thus, it was impossible to enhance theheat resistance to a desired level by increasing the amount of anantioxidant.

As another attempt, it was proposed to incorporate a specific kind of anantioxidant. For example, an acrylic rubber composition having nickeldialkyldithiocarbamate incorporated therein as an antioxidant has beenproposed in Japanese Unexamined Patent Publication No. S56-135539. Theacrylic rubber composition in this publication caused bloom only to anegligible extent after it was crosslinked, even in the case when theamount of nickel dialkyldithiocarbamate was increased, but, the heatresistance of a crosslinked object could not be enhanced to asufficiently high level. Especially the obtained crosslinked articletends to exhibit a large variation in elongation upon the imposition ofheat load.

As still another attempt, it was proposed to two kinds of aromaticsecondary amine antioxidants in an acrylic rubber to enhance the heatresistance (Japanese Unexamined Patent Publication No. H11-21411).However, the heat resistance of resulting crosslinked articles stillcould not be enhanced to a desired high level in some fields andespecially, the crosslinked articles exhibited undesirably largevariation in elongation depending upon the heat load applied. If theamount of the two kinds of secondary amine antioxidants is increased toenhance the heat resistance more, bloom is liable to occur.

DISCLOSURE OF THE INVENTION

A primary object of the present invention is to provide an crosslinkedacrylic rubber article having excellent heat resistance and especiallyexhibiting a reduced variation in elongation upon the imposition of heatload, as compared with the conventional acrylic rubber articles.

The present inventors have found that an acrylic rubber compositioncomprising an acrylic rubber containing units of an ethylenicallyunsaturated monomer having a carboxyl group, an aromatic secondary aminecompound and nickel dialkyldithiocarbamate gives a crosslinked articlehaving excellent heat resistance and exhibiting a reduced variationespecially in elongation upon the imposition of heat load. The presentinvention has been completed on the basis of this finding.

Thus, in accordance with the present invention, there are provided:

an acrylic rubber composition comprising 100 parts by weight of (a) anacrylic rubber having carboxyl groups, 0.3 to 8 parts by weight of (b)an aromatic secondary amine compound, and 0.3 to 8 parts by weight of(c) nickel dialkyldithiocarbamate.

a crosslinkable acrylic rubber composition comprising 100 parts byweight of (a) an acrylic rubber having carboxyl groups, 0.3 to 8 partsby weight of (b) an aromatic secondary amine compound, 0.3 to 8 parts byweight of (c) nickel dialkyldithiocarbamate, and 0.1 to 10 parts byweight of a crosslinking agent; and

a crosslinked article made by crosslinking the crosslinkable acrylicrubber composition.

BEST MODE FOR CARRYING OUT THE INVENTION

The acrylic rubber composition of the present invention comprises 100parts by weight of (a) an acrylic rubber having carboxyl groups, 0.3 to8 parts by weight of (b) an aromatic secondary amine compound, and 0.3to 8 parts by weight of (c) nickel dialkyldithiocarbamate.

The acrylic rubber (a) having carboxyl groups used in the presentinvention (which acrylic rubber (a) is hereinafter called “acrylicrubber (a)” for brevity when appropriate) contains a carboxyl grouppreferably in an amount of 5×10⁻⁴ to 4×10⁻¹ ephr, more preferably2.5×10⁻³ to 2×10⁻¹ ephr, and especially preferably 5×10⁻³ to 1×10⁻¹ephr. If the content of carboxyl groups in the acrylic rubber (a) havingcarboxyl groups is too small, the acrylic rubber composition cannot becrosslinked to a desirably sufficient extent and a crosslinked articlemade therefrom sometimes becomes difficult to retain its shape. Incontrast, if the content of carboxyl groups is too large, a crosslinkedarticle made therefrom sometimes becomes rigid and loses its rubberelasticity.

The acrylic rubber (a) may be a copolymer made by copolymerization of anacrylic acid ester monomer as the principal constituent with anethylenically unsaturated monomer having a carboxyl group or carboxylgroups, and an optional other copolymerizable monomer. Alternatively,the acrylic rubber (a) may be a modified copolymer made by modifying aconventional acrylic rubber prepared by copolymerization, to introducecarboxyl groups therein. The acrylic rubber (a) will be morespecifically described by the copolymer made by copolymerization of anacrylic acid ester monomer with an ethylenically unsaturated monomerhaving a carboxyl group or carboxyl groups.

As the acrylic acid ester monomer, an acrylic acid alkyl ester monomer,the alkyl group of which has 1 to 8 carbon atoms, can be used alone, or,preferably in combination with acrylic acid alkoxyalkyl ester, thealkoxyalkyl group of which has 2 to 16 carbon atoms.

As specific examples of the acrylic acid alkyl ester monomer having analkyl group of 1 to 8 carbon atoms, there can be mentioned methylacrylate, ethyl acrylate, n-propyl acrylate, n-butyl acrylate, isopropylacrylate, isobutyl acrylate, n-hexyl acrylate, 2-ethylhexyl acrylate andcyclohexyl acrylate. Of these, acrylic acid alkyl ester monomers havingan alkyl group of 2 to 4 carbon atoms are preferable. Ethyl acrylate andn-butyl acrylate are especially preferable.

As the acrylic acid alkoxyalkyl ester monomer having an alkoxyalkylgroup of 2 to 16 carbon atoms, acrylic acid alkoxyalkyl ester monomershaving an alkoxyalkyl group of 2 to 8 carbon atoms are preferable. Asspecific examples of the preferable acrylic acid alkoxyalkyl estermonomers having an alkoxyalkyl group of 2 to 8 carbon atoms, there canbe mentioned methoxymethyl acrylate, ethoxymethyl acrylate,2-ethoxyethyl acrylate, 2-butoxyethyl acrylate, 2-methoxyethyl acrylate,2-propoxyethyl acrylate, 3-methoxypropyl and 4-methoxybutyl acrylate. Ofthese, 2-ethoxyethyl acrylate and 2-methoxyethyl acrylate are especiallypreferable.

The ethylenically unsaturated monomer having a carboxyl group orcarboxyl groups is a monomer having an ethylenically unsaturated bondand containing a carboxyl group or carboxyl groups, or a carboxylicanhydride group.

As specific examples of the ethylenically unsaturated monomer having acarboxyl group or carboxyl groups, there can be mentioned carboxylicacid monomers such as acrylic acid, methacrylic acid, ethacrylic acid,itaconic acid, maleic acid, fumaric acid and citraconic acid;butenedioic acid monoalkyl ester monomers such as monomethyl maleate,monoethyl maleate, mono-n-butyl maleate, monomethyl fumarate, monoethylfumarate and mono-n-butyl fumarate; and carboxylic acid anhydridemonomers such as maleic anhydride and citraconic anhydride. Of these,butenedioic acid monoalkyl ester monomers are preferable. Monoethylmaleate, mono-n-butyl maleate, monoethyl fumarate and mono-n-butylfumarate are especially preferable.

The acrylic rubber (a) may contain, in addition to units of theabove-mentioned monomers, units of a monomer copolymerizable with theabove-mentioned monomers. As specific examples of the copolymerizablemonomer, there can be mentioned ethylene, propylene, acrylonitrile,methacrylonitrile, styrene, α-methylstyrene, vinyl chloride, vinylidenechloride, vinyl acetate, ethyl vinyl ether, butyl vinyl ether,divinylbenzene, furfuryl acrylate, acrylamide, ethylene glycoldiacrylate, propylene glycol diacrylate, alkoxyalkyl methacrylates,methacrylamide, ethylene glycol dimethacrylate, propylene glycoldimethacrylate, isoprene, butadiene, chloroprene, piperylene,dicyclopentadiene, norbornene, ethylidenenorbornene, hexadiene andnorbornadiene.

The total amount of units of the acrylic acid ester monomer as theprincipal constituent and units of the ethylenically unsaturated monomerhaving a carboxyl group or carboxyl groups in the acrylic rubber (a)having carboxyl groups is preferably at least 70% by weight, morepreferably at least 80% by weight and especially preferably at least 90%by weight, based on the weight of the copolymer. If the total amount ofthe acrylic acid ester monomer units and the carboxyl group-containingethylenically unsaturated monomer units is too small, namely, the amountof units of the monomer copolymerizable therewith is too large, theacryl rubber (a) tends to become poor in rubber characteristics such asrubber elasticity.

In the acrylic rubber (a), the amount of the acrylic acid ester monomerunits is preferably in the range of 90 to 99.5% by weight, morepreferably 95 to 99% by weight, based on the total amount of the acrylicacid ester monomer units and the units of the ethylenically unsaturatedmonomer having a carboxyl group or carboxyl groups or a carboxylicanhydride group. If the relative amount of the acrylic acid estermonomer units is too large, namely, the relative amount of the carboxylgroup-containing ethylenically unsaturated monomer units is too small, acrosslinkable rubber composition tends to become poor incrosslinkability. In contrast, if the relative amount of the acrylicacid ester monomer units is too small, namely, the relative amount ofthe carboxyl group-containing ethylenically unsaturated monomer units istoo large, a crosslinked article obtained from a crosslinkable rubbercomposition tends to have poor tensile strength and elongation.

The acrylic rubber (a) has a Mooney viscosity (ML₁₊₄, 100° C.)preferably in the range of 10 to 70, more preferably 20 to 60 andespecially preferably 30 to 50. If the Mooney viscosity is too small,the shapability and processability of a crosslinkable rubber compositionand the mechanical strength of a crosslinked article are liable to bepoor. In contrast, if the Mooney viscosity is too small, the shapabilityand processability of a crosslinkable rubber composition are liable tobe poor.

The aromatic secondary amine compound (b) used in the present inventionis a secondary amine having an aromatic ring or rings, and includes, forexample, diarylamines, diaryl-p-phenylenediamines,alkyl-aryl-p-pneylenediamines, di(arylamine)alkylenes anddialkyl-p-phenylenediamines. As the aromatic secondary amine compound(b), those which have a structure such that two groups each having anaromatic ring are bonded to a nitrogen atom or nitrogen atoms arepreferable. Of these, diarylamines, diaryl-p-phenylenediamines anddi(arylamine)alkylenes are preferable. The two groups each having anaromatic ring may be the same as or different from each other.

As specific examples of the diarylamine, there can be mentionedphenyl-α-naphthylamine, 4,4′-dioctyldiphenylamine,4,4′-bis(α,α-dimethylbenzyl)diphenylamine,p-(p-toluenesulfonylamide)diphenylamine, p-isopropoxy-diphenylamine,bis(phenylisopropylidene)-4,4-diphenylamine,4-(α-phenylethyl)diphenylamine and4,4′-bis(α-phenylethyl)-diphenylamine. As specific examples of thediaryl-p-phenylenediamine, there can be mentionedN,N′-diphenyl-p-phenylenediamine. N,N′-di-2-naphthyl-p-phenylenediamineand N,N′-bis(1,4-dimethylpentyl)-p-phenylenediamine. As specificexamples of the di(arylamine)alkylene, there can be mentionedN,N′-diphenylethylenediamine and N,N′-diphenyl-propylenediamine. Asspecific examples of the alkyl-aryl-p-phenylenediamine, there can bementioned N-isopropyl-N′-phenyl-p-phenylenediamine,N-cyclohexyl-N′-phenyl-p-phenylenediamine andN-phenyl-N′-(3-methacryloyloxy-2-hydroxypropyl)-p-phenylenediamine andN,N′-bis(1-methylheptyl)-p-phenylenediamine.

Of these, phenyl-α-naphthylamine, 4,4′-dioctyl-diphenylamine,4,4′-bis(α,α-dimethylbenzyl)diphenylamine,4-(α-phenylethyl)diphenylamine, 4,4′-bis(α-phenylethyl)-diphenylamine,N,N′-diphenyl-p-phenylenediamine,N-isopropyl-N′-phenyl-p-phenylenediamine andN-phenyl-N′-(3-methacryloyloxy-2-hydroxypropyl)-p-phenylenediamine arepreferable.

The amount of the aromatic secondary amine compound (b) is in the rangeof 0.3 to 8 parts by weight, preferably 0.4 to 5 parts by weight andespecially preferably 0.5 to 3, based on 100 parts by weight of theacrylic rubber (a). If the amount of the aromatic secondary aminecompound (b) is too small, a crosslinked article tends to have a poorheat resistance. In contrast, if the amount of the aromatic secondaryamine compound (b) is too large, the aromatic secondary amine compound(b) is liable to bleed on the surface of a crosslinked article.

The nickel dialkyldithiocarbamate (c) used in the present invention is acompound represented by the following general formula:

R¹R²NCSSNi

wherein R¹ and R² represents an alkyl group. The two alkyl groups may beeither the same as or different from each other. The alkyl group maybe acycloalkyl group. Each of the alkyl groups preferably has 1 to 8 carbonatoms, and, as specific examples of the alkyl group, there can bementioned methyl, ethyl, butyl, hexyl, octyl, 2-ethylhexyl, cyclohexyland cyclooctyl groups. As specific examples of the preferable nickeldialkyldithiocarbamate, there can be mentioned nickeldimethyldithiocarbamate, nickel diethyldithiocarbamate and nickeldibutyldithiocarbamate. Nickel diethyldithiocarbamate and nickeldibutyldithiocarbamate are especially preferable.

The amount of the nickel dialkyldithiocarbamate (c) is in the range of0.3 to 8 parts by weight, preferably 0.4 to 5 parts by weight andespecially preferably 0.5 to 3 parts by weight, based on 100 parts byweight of the acrylic rubber (a). If the amount of the nickeldialkyldithiocarbamate (c) is too small, the physical properties of acrosslinked article are liable to greatly vary depending upon the heatload imposed thereto. In contrast, if the amount of the nickeldialkyldithiocarbamate (c) is too large, the nickeldialkyldithiocarbamate tends to bleed on the surface of a crosslinkedarticle.

The crosslinkable acrylic rubber composition of the present inventioncomprises a crosslinking agent in addition to the ingredients in theabove-mentioned acrylic rubber composition.

The crosslinking agent used is not particularly limited provided that itis capable of crosslinking the acrylic rubber (a), and, as specificexamples thereof, there can be mentioned aliphatic diamine compoundssuch as hexamethylenediamine, hexamethylenediamine carbamate andN,N′-dicinnamylidene-1,6-hexanediamine; aromatic diamine compounds suchas 4,4′-methylenedianiline, m-phenylenediamine, 4,4′-diaminodiphenylether, 3,4′-diaminodiphenyl ether,4,4′-(m-phenylene-diisopropylidene)dianiline,4,4′-(p-phenylene-diisopropylidene)dianiline,2,2′-bis[4-(4-aminophenoxy)phenyl]propane, 4,4′-diaminobenzanilide and4,4′-bis(4-amoinophenoxy)biphenyl; polyfunctional isocyanate compoundssuch as hexamethylene diisocyanate, dimethyldiphenylene diisocyanate,isophoron diisocyanate, trimethylhexamethylene dilsocyanate, blockeddiisocyanate and dicyclhexylmethane diisocyanate; and hydrazidegroup-containing compounds such as isophthalic acid dihydrazide,terephthalic acid dihydrazide and phthalic acid dihydrazide. Of these,polyamine compounds such as aliphatic diamine compounds and aromaticdiamine compounds are preferable.

The amount of the crosslinking agent is in the range of 0.1 to 10 partsby weight, preferably 0.2 to 7 parts by weight and especially preferably0.3 to 5 parts by weight, based on 100 parts by weight of the acrylicrubber (a). If the amount of the crosslinking agent is too small, acrosslinked article has a greatly low mechanical strength and exhibitslarge variation in elongation and tensile strength depending upon theheat load imposed. In contrast, if the amount of the crosslinking agentis too large, a crosslinked article tends to have undesirably largehardness and low elongation and exhibit too small variation dependingupon the heat load.

In the present invention, if desired, a crosslinking accelerator can beused in combination with the crosslinking agent. The amount of thecrosslinking accelerator is preferably in the range of 0.1 to 20 partsby weight, more preferably 0.2 to 15 parts by weight and especiallypreferably 0.3 to 10 parts by weight, based on 100 parts by weight ofthe acrylic rubber (a). If the amount of the crosslinking accelerator istoo small, a substantial crosslinking acceleration effect is sometimesnot manifested, and a crosslinked article is liable to have an extremelylow mechanical strength and exhibit undesirably large variation inelongation and tensile strength depending upon the heat load imposed. Incontrast, if the amount of the crosslinking accelerator is too large,the crosslinking rate becomes too high upon crosslinking, thecrosslinking accelerator tends to bloom on the surface of a crosslinkedarticle, and the crosslinked article is liable to become too hard.

The crosslinking accelerator used is not particularly limited providedthat its is capable of accelerating a crosslinking reaction when it isused in combination with the crosslinking agent. As preferablecrosslinking accelerators used in combination with a polyamine compound,i.e., a preferable crosslinking agent, there can be mentioned bases andconjugated bases, which have a base dissociation constant in the rangeof 10⁻¹² to 10⁺⁶ in water at 25° C. and are substantially incapable ofreacting with a crosslinking monomer units to cause crosslinking. Asexamples of the preferable crosslinking accelerator, there can bementioned guanidine accelerators, quaternary onium salt accelerators,tertiary amine accelerators, tertiary phosphine accelerators, alkalimetal salts of a weak acid, alkali metal alkoxylates and alkali metalphenolates. The guanidine accelerators include, for example,1,3-diphenyl-guanidine and di-o-tolylguanidine. The quaternary oniumsalt accelerators include, for example, tetrabutylammonium bromide andtetrabutylammonium chloride. The tertiary amine accelerators include,for example, hexamethyltriethylene-tetramine and1,8-diaza-bicyclo[5,4,0]undecene-7. The tertiary phosphine acceleratorsinclude, for example, triphenylphosphine and tri(methylphenyl)phosphine.The alkali metal salts of a weak acid include, for example, sodium,potassium and lithium salts of inorganic weak acids such as phosphoricacid, carbonic acid and hydrogencarbonic acid, and organic weak acidssuch as stearic acid and lauric acid. The alkali metal alkoxylatesinclude, for example, sodium methoxide, sodium isopropoxide andpotassium isopropoxide. The alkali metal phenolates include, sodiumphenolate, potassium phenolate and potassium benzoate. Thesecrosslinking accelerators can also be used as a combination of at leasttwo thereof.

The crosslinkable rubber composition of the present invention can beprepared by mixing together the above-mentioned ingredients by anappropriate mixing means such as roll mixing, Banbury mixing, screwmixing or solution mixing.

If desired, additives can be incorporated in the acrylic rubbercomposition and crosslinkable acrylic rubber composition of the presentinvention, respecitively, provided that the intended characteristics ofthe compositions can be obtained. The additives include, for example,reinforcers, fillers, antioxidants and antioxidants other than thosementioned above, light stabilizers, scorch retarders, crosslinkingretarders, plasticizers, processing aids, lubricants, stickners,self-adhesives, fire-retarders, mildewproofing agents, antistaic agentsand colorants.

The acrylic rubber composition and crosslinkable acrylic rubbercomposition of the present invention can be prepared, respectively, bymixing together the above-mentioned ingredients by an appropriate mixingmeans such as roll mixing, Banbury mixing, screw mixing or solutionmixing.

No limitation is imposed to the methods of shaping and crosslinking thecrosslinkable rubber composition of the present invention, but thecrosslinkable rubber composition can be suitable for extrusion shaping.The shaping and crosslinking can be carried out concurrently, or,separately in this order, which can be determined depending upon theparticular shaping method, the particular crosslinking method, and theparticular shape of a crosslinked article to be produced.

The crosslinkable rubber composition of the present invention can becrosslinked by heating. The heating temperature for crosslinking ispreferably in the range of 130 to 220° C., more preferably 140 to 210°C. When the heating temperature is too low, a substantial longcrosslinking time is required or the crosslinking density is reduced. Incontrast, when the heating temperature is too high, crosslinking rapidlyoccurs and shaping failure is sometimes caused.

The crosslinking time varies depending upon the crosslinking method, thecrosslinking temperature and the shape of a crosslinked article to beproduced, but is preferably in the range of 30 seconds to 5 hours inview of the crosslinking density and the production efficiency.

The heating means conventionally adopted for crosslinking rubber canappropriately be used, which includes, for example, press heating, steamheating, oven heating and hot air heating.

The invention will now be specifically described by the followingexamples and comparative examples.

EXAMPLES 1-2 Comparative Examples 1-4

As acrylic rubber (a), acrylic rubber comprising 58% by weight of ethylacrylate units, 40% by weight of butyl acrylate units and 2% by weightof monobutyl fumarate units, and having a Mooney viscosity (ML₁₊₄, 100°C.) of 35 (hereinafter abbreviated to “acrylic rubber A”) was used. 100parts by weight of acrylic rubber A, 60 parts by weight of MAF carbonblack (“Seast 116” available from Tokai Carbon K.K.), 2 parts by weightof stearic acid, and the ingredients shown in Table 1 were kneadedtogether by using a Banbury mixer at 50° C., and then the mixture waskneaded together with 0.45 part by weight of 4,4′-diaminodiphenyl etheras a crosslinking agent, and 2 parts by weight of di-o-tolylguanidine(“Nocceler DT” available from Ohuchi Shinko Kagaku Kogyo K.K.) as acrosslinking accelerator by using an open roll to prepare acrosslinkable rubber composition.

This crosslinkable rubber composition was press-molded at 170° C. for 20minutes whereby molding and crosslinking were carried out concurrentlyto prepare a test sample having a size of 15 cm×15 cm×2 mm. The moldedtest sample was subjected to secondary crosslinking at 170° C. for 4hours. The tensile strength, elongation, hardness and permanent set ofthe crosslinked article were evaluated according to JIS K6301. Theresults are shown in Table 1.

The test sample was further left to stand at 175° C. for 500 hours, andthe tensile strength, elongation, hardness and permanent set of thecrosslinked article were evaluated according to JIS K6301. The resultsare shown in Table 1.

comparative examples 5-7

As acrylic rubber (a), acrylic rubber comprising 48.2% by weight ofethyl acrylate units, 30.0% by weight of butyl acrylate units, 20.0% byweight of 2-methoxyethyl acrylate units and 1.8% by weight of vinylchloroacetate units, and having a Mooney viscosity (ML₁₊₄, 100° C.) of40 (hereinafter abbreviated to “acrylic rubber B”) was used. Acrylicrubber A and the ingredients shown in Table 2 were kneaded together byusing a Banbury mixer at 50° C., and then the mixture was kneadedtogether with 0.5 part by weight of 2,4,6-trimercapto-s-triazine(“ZISNE-F” available from Sankyo Kasei Kogyo K.K.) as a crosslinkingagent, and 1.5 parts by weight of zinc dibutyldithiocarbamate (“NoccelerBZ” available from Ohuchi Shinko Kagaku Kogyo K.K.) as a crosslinkingaccelerator by using an open roll to prepare a crosslinkable rubbercomposition.

This crosslinkable rubber composition was molded and crosslinked, andfurther subjected to secondary crosslinking by the same procedures asdescribed in Examples 1 and 2 to prepare a crosslinked article. Thephysical properties of the crosslinked article were evaluated, andfurther the crosslinked article was further left to stand at 175° C. for500 hours, and the physical properties of the crosslinked article wereevaluated, by the same procedures as described in Examples 1 and 2. Theresults are shown in Table 2.

TABLE 1 Example Comparative Example 1 2 1 2 3 4 Composition ofcrosslinkable rubber composition (wt. parts) (a) Acrylic rubber A 100100 100 100 100 100 (b) N-phenyl-N′-isopropyl-p- — 2 — — 1 —phenylenediamine 4,4′-bis (α, α-dimethyl- 2 — — 3 2 — benzyl)diphenylamine (c) Nickel dibutyldithio- 1 2 2 — — 2 carbamate Poly(2,2,4-trimethyl-1,2- — — — — — 1 dihydroquinone Dry physicalproperties: Tensile strength (MPa) 13.1 13.0 13.3 13.2 13.3 13.3Elongation (%) 220 220 240 240 260 250 Hardness 69 70 70 68 68 68Physical properties after imposition of heat load Tensile strength (MPa)7.1 7.8 8.4 6.6 7.5 8.5 Elongation (%) 180 150 140 150 140 120 Hardness87 89 91 87 92 92 Variation of physical properties after imposition ofheat load Tensile strength variation (%) −46 −40 −37 −50 −44 −36Elongation variation (%) −18 −32 −42 −38 −46 −52 Hardness variation +18+19 +21 +19 +24 +24

In comparative Example 1 wherein an acrylic rubber composition notcontaining an aromatic secondary amine (b) was used, variation inphysical properties, for example, variation in elongation by heat loadwas too large. In comparative Example 2 wherein an acrylic rubbercomposition not containing nickel dialkyldithiocarbamate (c) was used,variation in physical properties, for example, variation in elongationby heat load was too large. In comparative Example 3 wherein an acrylicrubber composition containing only two kinds of aromatic secondaryamines (b), but not containing nickel dialkyldithiocarbamate (c) wasused, variation in physical properties, for example, variation inelongation and hardness by heat load was too large. In comparativeExample 4 wherein an acrylic rubber composition containing anamine-ketone antioxidant instead of aromatic secondary amine (b) wasused, variation in physical properties, for example, variation inelongation and hardness by heat load was too large.

TABLE 2 Example Comp. Example 1 2 5 6 7 Composition of crosslinkablerubber composition (wt. parts) (a) Acrylic rubber A 100 100 — — —Acrylic rubber B — — 100 100 100 (b) N-phenyl-N′-isopropyl-p- — 2 — — —phenylenediamine 4,4′-bis(α, α-dimethyl- 2 — 2 2 — benzyl)diphenylamine(c) Nickel dibutyldithio- 1 2 1 — 2 carbamate Poly(2,2,4-trimethyl-1,2-— — — — — dihydroquinone 4,4′-diaminodiphenyl ether 0.45 0.45 — — —Di-o-tolylguanidine 2.0 2.0 — — — 2,4,6-trimercapto-s-triazine — — 0.50.5 0.5 Zinc dibutyldithiocarbamate — — 1.5 1.5 1.5 Dry physicalproperties: Tensile strength (MPa) 13.1 13.0 11.7 11.4 11.4 Elongation(%) 220 220 170 170 170 Hardness 69 70 70 71 70 Physical propertiesafter imposition of heat load Tensile strength (MPa) 7.1 7.8 8.6 8.6 8.4Elongation (%) 180 150 80 60 40 Hardness 87 89 91 91 94 Variation ofphysical properties after imposition of heat load Tensile strengthvariation (%) −46 −40 −26 −25 −26 Elongation variation (%) −18 −32 −53−65 −76 Hardness variation +18 +19 +21 +20 +24

In comparative Example 5 wherein acrylic rubber B containing vinylchloroacetate as crosslinking monomer units was used, variation intensile strength and hardness by imposition of heat load is small, butvariation in elongation by heat load is too large.

In comparative Example 6 wherein acrylic rubber B was used and anacrylic rubber composition not containing nickel dialkyldithiocarbamate(c) was used, variation in elongation by heat load is too large.

In comparative Example 7 wherein acrylic rubber B was used and anacrylic rubber composition not containing aromatic secondary amine (b)was used, variation in elongation by heat load is further large.

In contrast, acrylic rubber composition of the present invention ischaracterized as exhibiting low variation in physical properties, forexample, in elongation and tensile strength by heat load.

INDUSTRIAL APPLICABILITY

The crosslinked rubber article of the present invention is characterizedas having excellent heat resistance and exhibiting especially a reducedvariation in elongation upon the imposition of heat load. Therefore, thecrosslinked rubber article has a wide application as rubber material infields including transportation facilities such as automobiles, generalequipment, and electronic and electric fields. More specifically, thecrosslinked rubber article is useful as seals, vibrating insulators,wire coverings, industrial belts, hoses, sheet, rolls, boots, or partsof these members.

What is claimed is:
 1. An acrylic rubber composition comprising 100parts by weight of (a) an acrylic rubber having carboxyl groups, 0.3 to8 parts by weight of (b) an aromatic secondary amine compound, and 0.3to 8 parts by weight of (c) nickel dialkyldithiocarbamate.
 2. Theacrylic rubber composition according to claim 1, wherein the content ofcarboxyl groups in said acrylic rubber is in the range of 5×10⁻⁴ to4×10⁻¹ ephr.
 3. The acrylic rubber composition according to claim 1,wherein said acrylic rubber (a) having carboxyl groups is a copolymermade by copolymerization of an acrylic acid ester monomer with anethylenically unsaturated monomer having a carboxyl group or carboxylgroups.
 4. The acrylic rubber composition according to claim 3, whereinsaid acrylic acid ester monomer is selected from acrylic acid alkylester monomers, the alkyl group of which has 1 to 8 carbon atoms, andcombinations of acrylic acid alkyl ester monomers, the alkyl group ofwhich has 1 to 8 carbon atoms, with acrylic acid alkoxyalkyl estermonomers, the alkoxyalkyl group of which has 2 to 16 carbon atoms. 5.The acrylic rubber composition according to claim 3, wherein saidethylenically unsaturated monomer having a carboxyl group or carboxylgroups is selected from acrylic acid, methacrylic acid, ethacrylic acid,itaconic acid, maleic acid, fumaric acid, citraconic acid, monomethylmaleate, monoethyl maleate, mono-n-butyl maleate, monomethyl fumarate,monoethyl fumarate, mono-n-butyl fumarate, maleic anhydride andcitraconic anhydride.
 6. The acrylic rubber composition according toclaim 3, wherein the total amount of units of the acrylic acid estermonomer and units of the ethylenically unsaturated monomer having acarboxyl group or carboxyl groups in said acrylic rubber having carboxylgroups is at least 70% by weight based on the weight of the copolymer.7. The acrylic rubber composition according to claim 6, wherein theamount of the acrylic acid ester monomer units is in the range of 90 to99.5% by weight based on the total amount of the acrylic acid estermonomer units and the units of the ethylenically unsaturated monomerhaving a carboxyl group or carboxyl groups.
 8. The acrylic rubbercomposition according to claim 1, wherein said aromatic secondary aminecompound (b) has a structure such that two groups each having anaromatic ring are bonded to a nitrogen atom or nitrogen atoms.
 9. Theacrylic rubber composition according to claim 8, wherein said aromaticsecondary amine compound (b) is selected from phenyl-α-naphthylamine,4,4′-dioctyldiphenylamine, 4,4′-bis(α,α-dimethylbenzyl)diphenylamine,4-(α-phenylethyl)diphenylamine, 4,4′-bis(α-phenylethyl)diphenylamine,N,N′-diphenyl-p-phenylenediamine,N-isopropyl-N′-phenyl-p-phenylenediamine andN-phenyl-N′-(3-methacryloyloxy-2-hydroxypropyl)-p-phenylenediamine. 10.The acrylic rubber composition according to claim 1, wherein the nickeldialkyldithiocarbamate (c) has two alkyl groups each having 1 to 8carbon atoms.
 11. The acrylic rubber composition according to claim 10,wherein the nickel dialkyldithiocarbamate (c) is selected from nickeldimethyldithiocarbamate, nickel diethyldithiocarbamate and nickeldibutyldithiocarbamate.
 12. A crosslinkable acrylic rubber compositioncomprising 100 parts by weight of (a) an acrylic rubber having carboxylgroups, 0.3 to 8 parts by weight of (b) an aromatic secondary aminecompound, 0.3 to 8 parts by weight of (c) nickel dialkyldithiocarbamate,and 0.1 to 10 parts by weight of a crosslinking agent.
 13. Thecrosslinkable acrylic rubber composition according to claim 12, whereinthe crosslinking agent is a polyamine compound.
 14. The crosslinkableacrylic rubber composition according to claim 13, wherein thecrosslinking agent is selected from hexamethylenediamine,hexamethylenediamine carbamate, N,N′-dicinnamylidene-1,6-hexanediamine,4,4′-methylenedianiline, m-phenylenediamine, 4,4′-diaminodiphenyl ether,3,4′-diaminodiphenyl ether, 4,4′-(m-phenylenediisopropylidene)dianiline,4,4′-(p-phenylenediisopropylidene)dianiline,2,2′-bis[4-(4-aminophenoxy)phenyl]propane, 4,4′-diaminobenzanilide and4,4′-bis(4-amoinophenoxy)biphenyl.
 15. The crosslinkable acrylic rubbercomposition according to claim 12, which further comprises 0.1 to 20parts by weight of a crosslinking accelerator, based on 100 parts byweight of the acrylic rubber having carboxyl groups.
 16. Thecrosslinkable acrylic rubber composition according to claim 13, whichfurther comprises 0.1 to 20 parts by weight, based on 100 parts byweight of the acrylic rubber having carboxyl groups, of a crosslinkingaccelerator selected from guanidine accelerators, quaternary onium saltaccelerators, tertiary amine accelerators, tertiary phosphineaccelerators, alkali metal salts of a weak acid, alkali metalalkoxylates and alkali metal phenolates.
 17. A crosslinked article madeby crosslinking the crosslinkable acrylic rubber composition accordingto claim
 12. 18. The crosslinked article according to claim 17, which isa seal, a hose, a vibrating insulator, or boots, or a part thereof.